Modular converters and in particular modular multi-level converters (M2LC) are used for synthesizing voltages to be supplied to a load, for example an electrical motor or an electrical grid. The modular structure of modular converters offers a number of advantages such as modularity, scalability, high output voltage and low output current distortions.
A standard approach to achieve closed-loop control of a modular converter is to divide the control problem into two hierarchical upper and lower stages or layers. The upper, first stage may be based on vector control with a modulator. The vector control scheme may operate in a dq reference frame. By manipulating the voltage reference, which is input to the modulator, closed-loop control of load currents may be achieved. For example, carrier-based pulse width modulation (PWM) or space vector modulation (SVM) may be used in a modulator. Circulating currents and/or an energy balance within converter branches may be addressed by adding additional control loops.
The lower control stage utilizes the redundancy in the converter states (i.e. groups of switching states that produce the same line to line voltage) in order to balance the capacitor voltages. The capacitor voltages may be sorted in an ascending/descending order of their voltage values. For a charging current the capacitors with the lowest voltages may be selected first, and conversely, the capacitors with the highest voltages may be prioritized for discharging currents.